Cameron and Boris have sent Britain hurtling towards the EU exit

In a dramatic shift, both the PM and the Mayor now speak of EU withdrawal as a reasonable option.

David Cameron said that UK withdrawal from the EU was "imaginable". Photograph: Getty Images.

When Michael Gove declared earlier this year that Britain should be prepared to leave the EU if Brussels refuses to return major powers to Westminster, his comments were viewed as extraordinary. But his words have now been all but echoed by the country's two most senior Conservatives: Boris Johnson and David Cameron.

Asked on Sunday on The Andrew Marr Show whether Britain should be prepared to walk away from the EU if it can't secure the concessions it wants, Johnson replied: "That is correct, that's absolutely correct ... I mean I don't think it would be the end of the world. Don’t forget that 15 years ago the entire CBI, British Industry, the City, everybody was prophesying that there’d be gigantic mutant rats with two or three eyes swarming out of the gutters, the sewers, to gnaw the faces of the remaining British bankers because we didn’t go into the euro."

Yesterday in the Commons, mindful of the evolving threat from UKIP, David Cameron said that withdrawal from the EU was "imaginable".

"We are in charge of own destiny, we can make our own choices."

There we have it. EU withdrawal is not something that no sensible person should countenance but one of a menu of plausible policy options. In November, Angela Merkel declared: "I cannot imagine Britain not being part of Europe." Cameron has made it clear that he can. Having conceded as much, it will now be significantly harder for him to persuade his MPs and the public that they should not vote in favour of withdrawal if and when a referendum is held.

When he finally makes his speech on Europe in mid-January, Cameron will announce that a Conservative government would hold a referendum offering voters a choice between a looser relationship and none at all, an effective in/out plebiscite. The Prime Minister's hope is that the concessions he will extract from Brussels in areas such as social policy and justice, will convince his party that the benefits of membership outweigh the costs. But it is doubtful whether Barrasso and co. will play ball. As Nick Clegg declared in his recent speech on Europe, "It is wishful thinking to suggest we could effectively give ourselves a free pass to undercut the Single Market, only to then renegotiate our way back in to the laws that suit us."

He rightly added: "And let’s be honest: many of the people who advocate repatriation are the same people who want us out of Europe – full stop." If Cameron wants to prevent them winning the debate, he needs to make an unambiguous and positive case for EU membership. So long as he is either unwilling or unable to do so, Britain will continue to hurtle towards the exit.